Electric/electronic parts such as transistors, capacitors and LSI packages tend to shorten the service life and deteriorate the reliability with the heat generated during operation. As measures for preventing Such drawbacks, it has been proposed to interpose a thermal conductivity sheet, which is superior in thermal conductivity and close-contact, between electric/electronic parts and a heat sink (cooling means), such as a heat radiating fin, thermally connected to the electric/electronic parts for dissipating the generated heat to the exterior through the thermal conductivity sheet.
The thermal conductivity sheet is generally manufactured by dispersing a thermal conductivity filler in a matrix resin and shaping the mixture into the form of a sheet. Silicone rubber, for example, is employed as the matrix resin, while boron nitride in the form of particles, plates and needles, for example, is employed as the thermal conductivity filler.
More specifically, the thermal conductivity sheet is manufactured using the above-exemplified materials of a thermal conductivity filler and a matrix resin by any of three primary methods below.
In the first method, a matrix resin (e.g., silicone rubber) and a thermal conductivity filler (e.g., boron nitride (BN)) are combined and mixed with each other to prepare a material mixture. The material mixture is then shaped into the form of a sheet by using rolls, a calender, an extruder or the like as with usual rubber materials. The shaped sheet is pressed and vulcanized.
In the second method, a matrix resin (e.g., silicone rubber) and a thermal conductivity filler (e.g., boron nitride) are mixed and diluted in a solvent. A resulting mixture is then formed into a sheet by a doctor
The sheet is dried, pressed and then blade process. vulcanized.
In the third method, a matrix resin (e.g., silicone rubber) of 100 weight parts and a thermal conductivity filler (e.g., boron nitride) of 200 or more weight parts are combined together to prepare a compound material containing the thermal conductivity filler at a high ratio. The material is mixed by using a closed type kneading machine such as a kneader to form a powdery rubber material. A predetermined amount of the powdery rubber material is filled in a mold for shaping into a sheet, following which the molded sheet is pressed and vulcanized.
FIG. 41 is a sectional view showing a structure of the conventional thermal conductivity sheet fabricated by any of the prior art manufacture methods described above. In a prior art thermal conductivity sheet 10, thermal conductivity fillers 12 are combined and distributed in a matrix resin 11 in a condition where the long axes of the thermal conductivity fillers 12 are oriented in the direction of plane of the thermal conductivity sheet 10 (the longitudinal direction thereof).
The thermal conductivity fillers 12 are oriented so longitudinally of the sheet because the fillers 12 are aligned in the direction of rolling or extrusion when the material mixture is rolled or extruded for shaping into the sheet.
The inventors have found that the thermal conductivity sheet fabricated by any of the prior art manufacture methods has a problem below. Since the thermal conductivity fillers 12 are oriented in the direction of sheet plane, there is a tendency that the adjacent thermal conductivity fillers 12 are contacted with each other and the thermal conductivity fillers 12 as a whole are substantially continuously extended in the direction of plane of the thermal conductivity sheet 10 (the longitudinal direction thereof). Accordingly, heat is easy to transmit in the direction of plane of the thermal conductivity sheet 10, but is hard to transmit in the direction of thickness of the thermal conductivity sheet 10. As a result, the thermal conductivity sheet has poor performance when it is used for the purpose of mainly utilizing its heat radiating characteristics in the direction of sheet thickness.
The inventors have also found the fact as follows. In a thermal conductivity sheet in which a large amount of brittle materials such as ceramics and metals are distributed as thermal conductivity filler in a soft matrix resin, the modulus of elasticity is increased and pliability is decreased. Therefore, when the thermal conductivity sheet is fitted to a part to be cooled, the thermal conductivity sheet 10 is difficult to deform following surface irregularities of the cooled part. This results in the problem that the thermal conductivity sheet 10 does not sufficiently closely contact with the surface of the cooled part, the thermal conductivity resistance is increased, and hence the heat radiating characteristics are deteriorated.
A thermal conductivity sheet disclosed in Japanese patent laid-open No. 54-163398 is formed of a composite of resin and boron nitride powder. The particle size of the boron nitride powder is set to be 0.2 to 1 time the sheet thickness, and the powder is filled into the sheet by pressing while it is also exposed to the sheet surface. Also, a thermal conductivity sheet disclosed in Japanese patent laid-open No. 3-20068 is of a structure that inorganic filler particles are arranged to lie continuously while contacting with each other. Any of the above-mentioned thermal conductivity sheets is intended to enhance thermal conductivity by increasing the density of fillers, which possess thermal conductivity, to such an extent that the fillers are present in the continuous form.
As a result of studies made by the inventors, however, it has been found that the thermal conductivity sheet lowers flexibility with an increase in a contact rate of the filler particles, and it eventually exhibits thermal conductivity as low as 4 W/m.multidot.K and slight flexibility and hence has a difficulty in drastically improving the thermal conductivity.
On the other hand, as disclosed in Japanese patent laid open No. 3-151658, there is known a thermal conductivity sheet in which small thermal conductivity fillers in the form of particles, plates, needles, etc. are oriented in the direction of sheet thickness such that the thermal conductivity fillers are contacted with each other, i.e., that the thermal conductivity fillers are arranged continuously with no resin layers therebetween.
In the above thermal conductivity sheet, however, since the thermal conductivity fillers are arranged continuously with no resin layers therebetween, flexibility (pliability) of the thermal conductivity sheet is impaired. Stated otherwise, in an attempt of increasing the density of the fillers to increase the thermal conductivity, it is inevitable for the thermal conductivity sheet becomes hard and brittle. Thus, the inventors have found that when the above thermal conductivity sheet is fitted to electronic/electric parts and heat sinks, the contact area is reduced, the contact thermal resistance is generated, and a sufficient degree of thermal conductivity cannot be obtained.
As another example in which thermal conductivity fillers are oriented in the direction of sheet thickness, a thermal conductivity member is disclosed in Japanese patent laid-open No. 62-240538. This thermal conductivity member is of a structure that metal short fibers or metal powders are planted or buried in an adhesive layer on a base sheet so as to form continuous heat radiating paths. When the metal short fibers are arranged in the direction of sheet thickness and the base sheet is made of an electrically conductive material, the electrically conductive material must be completely isolated by the adhesive layer in order that the thermal conductivity member has electrical insulation in its entirety. In this case, the thermal conductivity is improved by filling and dispersing metal powders in the adhesive layer.
As a result of studies made by the inventors, however, the following problems have been found. Because the amount of filled metal powders is restricted as a necessity to keep electrical insulation, the thermal conductivity is inevitably lowered. Also, the sufficiently large density of the filled metal powders entails a difficulty in ensuring flexibility. When the base sheet is made of the insulating materials disclosed in the above laid-open publication, the thermal conductivity of the thermal conductivity member as a whole is lowered and the effect of radiating and dissipating heat becomes insufficient. Further, when the thermal conductivity member, in which metal short fibers are planted in the adhesive layer on the base sheet, is fitted over an electric circuit, an electric trouble is apt to occur in that the metal short fibers may contact with each other to cause a short-circuit between a voltage applied portion and a ground potential portion on the circuit.
Additionally, as disclosed in Japanese patent laid-open No. 56-35494, there is known a thermal conductivity body in which a coating film comprising metal oxide particles dispersed in adhesive organic high molecules is formed on a highly thermally conductive base sheet. In this prior art, while the highly thermally conductive base sheet itself has sufficiently large thermal conductivity, the coating film consisted of a resin and harmless metal oxide particles except BeO dispersed in the resin has low thermal conductivity. Thus, it has been found from studies made by the inventors that the thermal conductivity body has low thermal conductivity as a whole and an effective heat radiating and dissipating action cannot be excepted.
Though the technical field is different from the present invention, various electrically conductive sheets having anisotropy are disclosed in Japanese patent laid-open No. 62-31909, No. 55-111014, No. 63-86322, No. 2-68811, No. 2-68812, for example. In any of the disclosed electrically conductive sheets, the sheet is given with anisotropy, while focusing on electric conductivity, by a structure that a conductive member is penetrated through the sheet in the direction of thickness thereof and the conductive member is exposed to the sheet surface. Stated otherwise, the disclosed electrically conductive sheets are intended to keep electric conductivity with high reliability, and are basically different in technical nature from the thermal conductivity sheet of the present invention which is intended to satisfy electrical insulation, thermal conductivity and flexibility at the same time.
Japanese patent laid-open No. 64-76608 discloses an electrically conductive member in which bumps are formed on an electrically conductive base material, and Japanese patent laid-open No. 1-286206 discloses an electrically conductive member in which a layer of a metal having the low melting point is formed on an electrically conductive portion. However, any of these prior parts has an object to ensure electrical connection of the electrically conductive member, and hence is different from the thermal conductivity sheet of the present invention aiming at thermal connection.
Furthermore, in the thermal conductivity sheet which is formed by arranging thermal conductivity fillers having the particle size of several .mu.m to 10 .mu.m such that the fillers lie continuously while contacting with each other with no resin layers therebetween, as disclosed in the above-cited Japanese patent laid-open publication (No. 3-20068), flexibility (pliability) of the thermal conductivity sheet is impaired. Therefore, the contact area of the thermal conductivity sheet with respect to electronic/electric parts and heat sinks (cooling means) is reduced and the contact thermal resistance is increased. Consequently, a sufficient degree of thermal conductivity cannot be expected.
In this way, as the dispersion density of thermal conductivity fillers is increased, the thermal conductivity is improved, but the flexibility of the thermal conductivity sheet is lowered. Then, it is difficult to obtain a thermal conductivity sheet which satisfies both high thermal conductivity and good flexibility, by any compounding techniques described above as the prior art methods.
With remarkable development of electronic/electric parts in recent years, an increase in integration, speed and output of electronic equipment, including semiconductor devices, is progressed and, correspondingly, the amount of heat generated from heat generating parts such as semiconductor devices is also increased. There is thus a demand for a thermal conductivity sheet which is more superior in heat radiating characteristics.
The present invention has been accomplished with a view of solving the problems described above, and its object is to provide a thermal conductivity sheet which is markedly superior in heat radiating characteristics (thermal conductivity) in the direction of sheet thickness, electrical insulation, and close-contact with respect to parts to be cooled.